1. Field
The present disclosure relates to a phosphor film, a method of forming the same, and a method of coating a phosphor layer on light-emitting chips.
2. Description of the Related Art
Light emitting diodes (LEDs) are semiconductor devices that produce light of various colors from a light emitting source formed at a PN junction of a compound semiconductor. Recently, blue LEDs and ultraviolet (UV) LEDs have been developed by using nitrides with excellent physical and chemical properties, and also, white light or light of other mono-color has been generated by using blue or UV LEDs and a phosphor material, thereby widening the application range of LEDs.
Conventionally, a LED chip includes a lower portion, an upper portion, and front, rear, left, and right side portions. The lower portion is formed of a reflective layer, and light is emitted through the upper portion and front, rear, left, and right side portions. In this structure, when color of an emitted light is changed by using a phosphor, it is very important to maintain a coating thickness of the phosphor on the top portion and four side surfaces of the LED chip to a constant level so as to obtain uniform light quality. In response to such a need, various conformal coating techniques have been introduced. However, most of the conformal coating techniques are performed before wire bonding and are applicable only to flipchip-type packages. Also, an additional process is required to open a separate wire bonding pad unit while coating a phosphor. In addition, when the phosphor coating is performed on a separate chip in a pre-mold lead frame-type packaging process, the methods may not be used.
An example of a packaging method in which a phosphor is coated after wire bonding is a dispensing process in which phosphor particles are dispersed in a light-transmissible resin with a high resistance to heat so as to form a paste and then a small amount of the paste is ejected through a nozzle having a given diameter to completely cover a chip with the phosphor. Regarding the dispensing process, it is important to appropriately adjust the viscosity of phosphor particles and the resin in order to eject an appropriate amount of small droplets. In this case, however, due to such a low viscosity to proceed with the process, solid inorganic phosphor particles may precipitate in a tube for storing materials, and also, they may precipitate even after the dispensing and before the phosphor paste is cured, and thus, dispersion of phosphor particles in the resin may be lowered. Correspondingly, a light quality distribution range may be widened.